The invention relates to a method of manufacturing an optical cable element. In the method, a synthetic resin envelope is extruded around one or more optical fibers at elevated temperature and the element is then cooled and wound on a reel.
Such a method is described in U.S. Pat. No. 4,414,165. According to this known method, a loose synthetic resin envelope (coating) is extruded around an optic fiber. After extrusion, the envelope is cooled to a temperature which is higher than room temperature. At this temperature, the cable element is reeled. While the element is wound, the optical fiber is braked. As a result, the fiber will be situated at the smallest radius of curvature within the envelope. With respect to the envelope the optical fiber then has an underlength.
The optical cable element is then further cooled to room temperature. As a result of the thermal contraction of the envelope, the length of the optical fiber increases with respect to that of the envelope. It is possible to determine and adjust the relative length of the fiber with respect to the envelope by determining and adjusting the degree of contraction of the envelope. As a result of this, for example, the previously adjusted underlength can be compensated or even overcompensated so that optical cable elements are obtained whose fibers have overlengths.
It is known that the optical transmission properties of optical cable elements depend very considerably on the overlength of the optical fibers and therefore on the reversible expansion and contraction ranges of the fiber envelope. For example, when the overlength is too large, the fiber is forced against the inner wall of the envelope after the envelope undergoes only a small contraction. As a result, the damping may increase considerably.
When the overlength is too small, the same problem will occur after the envelope undergoes a small expansion. The expansion and contraction ranges of the envelope, in which optical ratios are substantially constant, is also determined by the adjusted overlength. An adjusted overlength is therefore required to be maintained as much as possible after the manufacture of the element.
It has been found that in optical cable elements in which the overlength is adjusted by the reversible thermal contraction of the envelope, this requirement cannot substantially be fulfilled. This is particularly so when after the manufacture of the optical cable elements they are exposed to comparatively high temperatures, for example in further processing steps or as a result of intensive radiation by the sun upon storage in the open air. As a result of the influence of the temperature, the overlengths of the optical fibers changes as compared with the initially adjusted overlength. This change is not fully compensated when the temperature decreases, so that the optical transmission properties vary uncontrollably.